The present invention relates to porous materials of thermoplastic polymers made by sintering thermoplastic polymer particles.
Porous products of thermoplastic polymers and the like, which are commonly called porous plastics, are structures featuring interconnected (i.e. open cell) omnidirectional pores. The size of the pores typically is in the range of 5 microns to 500 microns. The structures are in a wide variety of product configurations for wicking, venting, filtering, sparging, etc. Porous plastic products include such things as nibs, catheter vents, compressed air filters, water filters, and waste water treatment diffusers.
Porous plastics are made by a form of sintering. Sintering is the process of fusing discrete particles by heat, with or without pressure, to form a porous structure. The sintering process uses raw material in the form of discrete particles of a thermoplastic polymer or the like.
A significant problem has been that some polymers are more difficult to sinter than others. It is believed that some difficult-to-sinter polymers, such as low density polyethylene and polyurethane, might have been impossible to sinter heretofore. Furthermore, polymers are primarily sold in the form of pellets, which are typically rod or cylindrical shaped nuggets of polymer xe2x85x9 to xc2xc inch in diameter. The pellets are too big to be sintered into structures with pore sizes suitable for most applications. The pellets can be ground to get raw materials of desired particle sizes, but such grinding is difficult and costly. The few plastics commercially available in flake or powder form constitute the bulk of materials used for sintering plastics.
The properties of some porous plastic products are not entirely satisfactory to perform the functions for which the products are used. For example, applicator heads of porous plastics, used on applicators for antiperspirants and deodorants in the form of fluent materials including creams, gels and the like, are somewhat abrasive. As a result, some users experience skin irritation. As another example, porous plastic is often used as a bubbler or diffuser to oxygenate water in wastewater treatment. Air is forced under pressure through the pores of a submerged hollow tube having a wall of porous plastic. The air exits the outer surface of the tube wall as bubbles that float up through the water. Oxygen diffuses into the water around each bubble. Traditional porous plastic bubblers have a broad range of pore sizes and thus emit a broad range of bubble sizes. However, the emission of large bubbles limits the efficiency of the gas transfer. Selective filtration is another example of an application where a broad pore size distribution is a limitation. Selective filtration refers to situations wherein one wishes to filter or exclude particles of a specific size but not exclude slightly smaller particles. This is difficult with a porous medium having a broad range of pore sizes.
By the present invention, the sinterability of polymers is improved by using as particles pellets smaller than typical size, that is, smaller than xe2x85x9 inch in diameter, especially by using micropellets having a diameter of about 0.060 inch or less. The pellets are obtained by underwater pelletizing of the polymers. As an example of the improvement, a low density polyethylene has been found to be quite easy to sinter after underwater micropelletlzing of the polyethylene.
In the underwater pelletizing process, an extrudate of a polymer moves from a die into water contacting the die, where the extrudate is cut and cooled to a solid state almost instantaneously. Alternatively, the pellets may be formed by water ring pelletizing. This process is similar to the underwater pelletizing process, but water does not contact the die. Instead, the extrudate is a cut at the die face by rotating blades and immediately flung into a quenching water ring surrounding the die. In both underwater pelletizing and water ring pelletizing, the extrudate is cut at the point of extrusion. These two processes are generically called rapid water quenched pelletizing.
The dimensions of a suitable pellet along three mutually perpendicular axes, one of which can be considered to be the diameter, are generally equal to one another and, thus, these pellets are closer to spherical than particles conventionally used. Such pellets are substantially the same size are uniform in shape, far more uniform than ground particles and are imporous. As a result, the porous plastic products produced from sintering pellets are denser than porous plastic products previously produced by sintering.
Products made by sintering rapid water quenched pellets of thermoplastic polymers and the like have smoother surfaces than products made by sintering other particles of plastics. As a result, antiperspirant or deodorant applicator heads made by sintering rapid water quenched pellets do not irritate the skin of users. This is due to the fact that the shape of the individual particles is more like a sphere than are the shapes of particles which have previously been sintered.
Pellets, especially micropellets and rapid water quenched pellets, which can be micropellets, form a sintered part with an exceptionally narrow range of pore sizes. This is a distinct advantage in some applications. For example, in bubblers of porous plastic made by sintering rapid water quenched micropellets, there is such a uniform pore structure that it is possible to eliminate the production of inefficient large bubbles. As another example, the porous plastic filter of sintered rapid water quenched micropellets according to the present invention has a relatively small range of pores and, thus, is well suited for excluding from liquids-and gases particles of a specific size but not of a slightly smaller size in sharply selective filtration.
Additional advantages of porous plastic materials made from sintered rapid water quenched pellets, specifically micropellets, are that the material has greatly increased tensile strength compared to prior art materials, the material is substantially less susceptible to creep under stress than prior art materials, the material has greater solvent resistance than prior art materials, the material provides a greater pressure drop in fluid flowing through the material, and parts made from the material have less size variation than parts made from prior art materials.